This invention relates in general to television signal transmission systems and specifically to a means and method for making television signal transmission systems more secure.
The above-mentioned '353 patent describes and claims a phase modulated television signal scrambling system in which information in the horizontal blanking intervals is suppressed in amplitude and the video carrier is subjected to a phase reversal. Copending application Ser. No. 712,949 describes and claims apparatus for transmitting data by modulation of the width of the horizontal blanking intervals. Application Ser. No. 025,236, discloses and claims a system for simultaneously encrypting and error protecting the data. In application Ser. No. 025,240 a novel frequency and phase locked loop is described for readily recovering the data in a phase modulated transmission system such as that disclosed in Ser. No. 712,949. The present invention is specifically directed to a means and method for utilizing a start signal (herein referred to as a framing code) for not only initiating the data detection system in the subscriber terminal but, in conjunction with a mode definition that is transmitted in encrypted form, for determining the descrambling mode to be used for descrambling the scrambled television signal.
In subscription television systems, signals are transmitted, either "over the air" or over a suitable cable network, in a scrambled form which renders the broadcast video information substantially unviewable when received by conventional television receivers. To permit viewing, each subscriber terminal is equipped with a decoder for descrambling the scrambled signals. Security is of major importance in designing a scrambling system to prevent unauthorized use of the broadcast information by non-paying subscribers. This need must, however, be tempered by economics and the need to be capable of reliably decoding the scrambling signals by authorized system subscribers.
It is well-known that the cable head-end or broadcasting station has the capability of individually communicating with subscriber terminals and downloading authorization information into suitable memories located thereat. The authorization information determines the types of programs the subscriber terminal is authorized to descramble. While elaborate scrambling techniques can readily frustrate potential "pirates," the descrambling means employed should be simple and economical to permit acceptance by cable system operators.
The phase modulation system described in the '353 patent provides a scrambled television signal. Use of the techniques described in the various copending applications significantly enhances the system security by additionally providing encrypted data that must be decrypted to permit proper operation of the descrambling apparatus at the subscriber terminal. The present invention adds still further protection to the scrambling system by incorporating different scrambling modes and providing a combination of a relatively noise-immune, but somewhat non-secure, start signal or framing code and relatively secure, but somewhat noise sensitive, encrypted data that determines which descrambling mode is to be used at any particular time. In the data packet communication system employed in the invention, the scrambling mode definitions are generally changed at random and communicated to individual subscriber terminals as encrypted data. Framing code signals are sent to permit rapid switching between the descrambling modes defined by the previously communicated encrypted data. As will be seen, this further enhanced security is obtained at substantially no added cost to the system.